1. Field of the Invention
The present invention relates to an optical pickup apparatus and an optical disc apparatus including the same.
2. Description of the Related Art
An optical disc is inserted into an optical disc apparatus (not shown) including an optical pickup apparatus. The optical disc inserted into the optical disc apparatus not shown is formed in a substantially circular plate shape.
Such discs includes: data read only optical discs such as a “CD-ROM”, “DVD-ROM”, “HD DVD-ROM”, and “BD-ROM”; data recordable optical discs such as a “CD-R”, “DVD-R”, “DVD+R”, “HD DVD-R”, and “BD-R”; data writable/erasable or data rewritable type optical discs such as a “CD-RW”, “DVD-RW”, “DVD+RW” (registered trademark), “DVD-RAM”, “HD DVD-RW”, “HD DVD-RAM”, and “BD-RE”, etc., for example.
To describe the optical discs, “CD” is an abbreviation of “Compact Disc” (trademark). “DVD” (registered trademark) is an abbreviation of “Digital Versatile Disc”. “HD DVD” (registered trademark) is an abbreviation of “High Definition DVD”. “Blu-ray” of “Blu-ray Disc” (registered trademark) means blue-violet laser which is employed for achieving higher-density recording as compared with red laser which is used to read/write signals for DVD. The “HD DVD” is made compatible with existing DVD series and has a storage capacity greater than that of the existing DVD series. The infrared laser has been used for the CD. The red laser has been used for the DVD. Whereas, the blue-violet laser is used when reading data/information/signals recorded in the “Blu-ray Disc” or “HD DVD” optical disc or writing data/information/signals into the “Blu-ray Disc” or “HD DVD” optical disc.
“ROM” of the “CD-ROM”, “DVD-ROM”, and “HD DVD-ROM” is an abbreviation of “Read Only Memory”. “BD-ROM” is an abbreviation of “Blu-ray Disc-ROM”. The “CD-ROM”, “DVD-ROM”, “HD DVD-ROM”, and “BD-ROM” are data/information read only discs. “R” of the “CD-R”, “DVD-R”, “DVD+R”, and “HD DVD-R” is an abbreviation of “Recordable”. “BD-R” is an abbreviation of “Blu-ray Disc-R”. The “CD-R”, “DVD-R”, “DVD+R”, “HD DVD-R”, and “BD-R” are data/information recordable discs. “RW” of the “CD-RW”, “DVD-RW”, “DVD+RW”, and “HD DVD-RW” is an abbreviation of “Re-Writable”. “BD-RE” is an abbreviation of “Blu-ray Disc-RE”. The “CD-RW”, “DVD-RW”, “DVD+RW”, “HD DVD-RW”, and “BD-RE” are data/information rewritable discs. “RAM” of the “DVD-RAM” and “HD DVD-RAM” is an abbreviation of “Random Access Memory”. The “DVD-RAM” and “HD DVD-RAM” are data/information writable/erasable discs.
An optical disc capable of recording data/information/signals in an optical disc apparatus includes a signal layer, which is a signal recording surface of the optical disc, provided with grooves (not shown) for storing data/information/signals. The grooves mean elongated dent portions, for example. In plan view of a circular optical disc, the grooves are formed in a substantially spiral shape. When a laser beam is applied to an optical disc, if the optical disc is viewed from the side of the signal layer received with the laser beam, the grooves are in a whorl shape. Since the grooves are very minute, the grooves are visually unrecognizable.
The optical pickup apparatus includes an optical system for detecting error signals such as a focus error signal and a tracking error signal so as to appropriately apply an irradiation spot onto a predetermined record track within an optical disc by controlling a position of an objective lens.
The focus means a focal point or focal spot, for example. Focusing means bringing into focus or being brought into focus, for example. The tracking means to track and observe a signal layer of an optical disc or minute pits (holes, dents), grooves (grooves), wobble (meandering), etc., on the signal layer of the optical disc with the use of light, to determine a position on a spirally shaped track, for example. The pit means a hole or a dent portion, for example. The wobble means meandering of a track on which data signals such as information, for example, are recorded.
A focusing detection method for an irradiation spot on an optical disc in the optical pickup apparatus includes a detection method based on a differential astigmatic method, for example. The differential astigmatic method is, for example, a method of detecting displacement of an irradiation spot by detecting distortion of a point image formed by an optical system having astigmatism. A tracking detection method for an irradiation spot on an optical disc in the optical pickup apparatus includes a detection method based on a differential push-pull method, for example. The differential push-pull method is, for example, a method of detecting displacement of an irradiation spot by a main beam for reading/writing data and two sub-beams for detecting a positional deviation correction signal.
To describe tracking error signal detection methods, for example, when detection of a tracking error signal is performed by the optical pickup apparatus for an optical disc of the CD standard (CD-ROM, CD-R, CD-RW, etc.) having a track pitch of 1.6 μm (microns/micro meters), for example, a “three-beam method (referred to as a three-spot method as well)” using three beams is primarily employed as a tracking error signal detection method. When detection of a tracking error signal is performed by the optical pickup apparatus for an optical disc of the DVD standards (DVD-ROM, DVD-R, and DVD-RW, etc.) having a track pitch of 0.74 μm, for example, an “in-line method” using at least three beams is primarily employed as a tracking error signal detection method. The designation of the tracking error signal detection methods here is given for convenience.
The track pitch of the DVD-RAM, DVD-ROM, DVD-R, DVD-RW, etc., of Version 1 is defined as substantially 0.74 μm, whereas the track pitch of the DVD-RAM of Versions 2.0 and 2.1 is defined as substantially 0.615 μm. As above, the track pitches are different between the DVD-RAM, DVD-ROM, DVD-R, DVD-RW, etc., of Version 1 and DVD-RAM of Versions 2.0 and 2.1.
First, there will be described the “three-beam method” primarily employed for the error signal detection in the CD standards. As shown in FIG. 17, the optical pickup apparatus includes a CD diffraction grating 320 on a light path between a semiconductor laser element 210 and a polarizing beam splitter 230. The CD diffraction grating 320 has linear grating grooves carved at even intervals in a given period and has a function of diffracting and splitting the laser beam emitted from the semiconductor laser element 210 into a total of three beams, which are a main beam (0th order light) and two sub-beams (±1st order diffracted light beams).
As a result of the three beams having passed through the polarizing beam splitter 230, a collimating lens 240, and an objective lens 250, a main spot 100 corresponding to the main beam and sub-spots 101 and 102 corresponding to two respective sub-beams are formed on a signal layer Da of an optical disc D, as shown on the left side of FIG. 18. On the signal layer Da of the optical disc D, a track D100 is periodically provided for recording signals, and an interval δ in a disc radial direction among the main spot 100 and the sub-spots 101 and 102 is adjusted to be identical to substantially one half of a periodic distance Dtp of the track D100 by a means of rotating the CD diffraction grating 320 around a light axis to be adjusted, etc. The reflected lights of the main spot 100 and the sub-spots 101 and 102 arrive again at the objective lens 250, the collimating lens 240, and the polarizing beam splitter 230, and a portion of a light amount of the lights passes through the polarizing beam splitter 230, and thereafter, is made incident on a photodetector 270 through a detection lens 260.
As shown on the right side of FIG. 18, the photodetector 270 is disposed with light-receiving surfaces 200a, 200b, and 200c respectively corresponding to the reflected lights of the main spot 100 and the sub-spots 101 and 102. When the reflected lights of the main spot 100 and the sub-spots 101 and 102 are made incident on the light-receiving surfaces 200a, 200b, and 200c, respectively, a main detection light spot 200 corresponding to the main spot 100 and sub-detection light spots 201 and 202 corresponding to the sub-spots 101 and 102 are formed.
If the main spot 100 performs scanning accurately on the track D100, the sub-detection light spots 201 and 202 have the same light amount. However, if the scanning of the main spot 100 is deviated from the track D100, a difference is generated between the light amounts of the sub-detection light spots 201 and 202. Therefore, a tracking error signal indicative of scanning deviation of the tracking is generated by executing a subtracting process with a subtractor 400 with respect to the light amounts of the sub-detection light spots 201 and 202, for example.
The “in-line method” primarily employed for the error signal detection in the DVD standard will then be described. The optical system of the in-line method is able to detect a tracking error signal basically based on substantially the same optical system as that of the three-beam method. However, as compared to the optical system of the three-beam method, a difference is that a DVD diffraction grating 340 is used in which a periodic structure of grating grooves formed on one half plane surface 341 is displaced in phase by about 180 degrees relative to a periodic structure of grating grooves formed on the other half plane surface 342, as shown on the left side of FIG. 20.
Here, it is assumed that the DVD diffraction grating 340 is provided at substantially the same location as the CD diffraction grating 320 shown in FIG. 17 replacing the CD diffraction grating 320. To support the in-line method, it is assumed that positions at which the DVD diffraction grating 340, a light converging optical system, etc., are disposed are adjusted such that the main spot 100 and the sub-spots 101 and 102 applied to the signal layer Da of the optical disc D are applied onto the same track D100, as shown on the left side of FIG. 19.
When the DVD main beam forming the main detection light spot 200 is applied to the light-receiving surface 200a of the photodetector 270, a subtractor 500a connected to the light-receiving surface 200a performs an operation to obtain a difference between output signals from the light-receiving surface 200a, to be generated as a main push-pull signal Sa, for example.
When a first DVD sub-beam forming the sub-detection light spot 201 is applied to the light-receiving surface 200b of the photodetector 270, a subtractor 500b connected to the light-receiving surface 200b performs an operation to obtain a difference between output signals from the light-receiving surface 200b, to be generated as a preceding sub-push-pull signal Sb, for example.
When a second DVD sub-beam forming the sub-detection light spot 202 is applied to the light-receiving surface 200c of the photodetector 270, a subtractor 500c connected to the light-receiving surface 200c performs an operation to obtain a difference between output signals from the light-receiving surface 200c, to be generated as a delayed sub-push-pull signal Sc, for example.
As shown on the right side of FIG. 19, the push-pull signal Sa detected from the main detection light spot 200 is output with a phase opposite to phases of the push-pull signals Sb and Sc detected from the sub-detection light spots 201 and 202 respectively corresponding to the sub-spots 101 and 102, as is the case with the three-beam method. Subsequently, an adder 510 adds the push-pull signals Sb and Sc, and a subtractor 530 subtracts the above added signals from the push-pull signal Sa, to be able to generate a tracking error signal with offset components of the push-pull signals Sa, Sb, and Sc canceled.
Recently, proposals have been made for an optical pickup apparatus capable of recording/reproduction of both the CD-standard optical discs and the DVD-standard optical discs. To achieve the cost reduction through simplification of the optical system, the optical pickup apparatus uses a multi-laser unit including: a CD semiconductor laser element for emitting a first laser beam having a first wavelength of 765 nm to 805 nm (nanometers) in the infrared waveband suitable for the CD standard; and a DVD semiconductor laser element for emitting a second laser beam having a second wavelength of 645 nm to 675 nm in the red waveband suitable for the DVD standard.
To further simplify the optical system, the optical pickup apparatus uses a two-wavelength supported diffraction grating supporting both the three-beam method for the CD standard and the in-line method for the DVD standard (see e.g., Japanese Patent Application Laid-Open Publication No. 2007-164962 (page 1, FIGS. 1 to 8)). For example, a two-wavelength supported diffraction grating 300A is configured such that the CD diffraction grating 320 is fixed to one surface and the DVD diffraction grating 340 is fixed to the other surface, in the one surface and the other surface opposed in a thickness direction of an optical glass plate 360 as shown in FIG. 20.
Other than the structure of the two-wavelength supported diffraction grating 300A shown in FIG. 20, for example, a two-wavelength supported diffraction grating 300B has been proposed that has a structure shown in FIG. 21 (see e.g., Japanese Patent Application Laid-Open Publication No. 2007-149249 (page 1, FIGS. 1 to 7)). For example, the two-wavelength supported diffraction grating 300B is configured such that the DVD diffraction grating 340 and the CD diffraction grating 320 including a liquid crystal material, etc., are fixed in superposed relation and thereafter, the fixed gratings are sandwiched between two optical glass plates 361 and 362 to be fixed.
However, in a case where the two-wavelength supported diffraction grating 300A or 300B made up by combining the CD diffraction grating 320 and the DVD diffraction grating 340 as above is used, for example, when a first laser beam of the CD standard is made incident on the CD diffraction grating 320, the first laser beam is diffracted by the CD diffraction grating 320 and split into three beams, which are a main beam (0th order light) and two sub-beams (±1st order diffracted light beams). The three beams are further diffracted and branched by the DVD diffraction grating 340.
Since the first laser beam or the second laser beam emitted from the multi-laser unit passes through both the CD diffraction grating 320 and the DVD diffraction grating 340 of the two-wavelength supported diffraction grating 300A or 300B as above, diffraction and splitting occur in each of the CD and DVD diffraction gratings 320 and 340, resulting in unnecessary diffracted light being generated. As a result, a problem occurs that detection accuracy of the error-signal such as the tracking error signal, etc., deteriorates.
For example, due to the generation of the unnecessary diffracted light, the transmission rates of the diffraction gratings 320 and 340 for the 0th order light and the ±1st order diffracted light beam deteriorates and, as a result, a problem occurs that utilization efficiency deteriorates in the outgoing light emitted from the multi-laser unit.
To enable support for a plurality of types of optical discs D having different track pitches Dtp such as a DVD-ROM, DVD-R, DVD-RW, DVD-RAM (versions 1, 2.0, and 2.1) without trouble, a market requires an advanced optical pickup apparatus in which control such as tracking control can easily be performed and an optical disc apparatuses including the advanced optical pickup apparatus in which the control such as the tracking control can easily be performed.
For example, the market requires an optical pickup apparatus in which an offset of the error signal such as a tracking error signal does not remain and an optical pickup apparatus in which amplitudes of the error signals such as a tracking error signal does not deteriorate with displacement of the objective lens 250, when recording/reproducing data into/from a plurality of types of optical discs D having different track pitches Dtp.
An inexpensive optical pickup apparatus and an inexpensive optical disc apparatus are required along with solutions to the above problems.